τ−μflavor violation as a probe of the scale of new physics

Motivated by the recent strong experimental evidence of large ${\nu }_{\mu }-{\nu }_{\tau }$ neutrino mixing, we explore current bounds on the analogous mixing in the charged lepton sector. We present a general formalism for dimension-6 fermionic effective operators involving $\tau -\mu$ mixing with a typical Lorentz structure $\left(\overline{\mu }\Gamma \tau \right)\left(q^{\alpha }\Gamma q^{\beta }\right),$ and discuss their relationship to the standard model gauge symmetry and the underlying flavor dynamics. We derive the low-energy constraints on the new physics scale associated with each operator, mostly from current experimental bounds on rare decay processes of $\tau ,$ hadrons or heavy quarks. For operators involving at least one light quark $\mathrm{}(u,d,s),$ these constraints typically give a bound on the new physics scale of a few TeV or higher. Those operators with two heavy quarks turn out to be more weakly constrained at the present, giving bounds of a few hundred GeV. A few scalar and pseudoscalar operators are free from all current experimental constraints.